North America, according to statistics published by the IEEE Insulated Conductors Committee, more than 32,000 km (19,884 miles) of low-voltage underground cables are installed annually. However, an Insulated Conductors Committee survey confirmed that some two-thirds of U.S. utilities are concerned about the reliability of their low-voltage underground cable networks.
The foremost cause of cable failures on the Hydro-Québec low-voltage underground cable system, where cables are installed in ducts, is attributable to mechanical damage, likely dating back to when the cable was installed. The damaged insulation in a conducting medium, such as contaminated water, may result in the development of an electrical arcing fault. This may then damage adjacent cables or equipment, jeopardize the security of the utility network and compromise public safety.
In certain conditions, the gases generated by the materials adjacent to the arcing fault can even result in violent explosions. A number of such major events occur in North America annually. Although these events are relatively infrequent on Hydro-Quebec’s low-voltage distribution system, their consequences could be serious for public safety and for the installations. The protection systems installed on the network, such as fuses and circuit breakers, are ineffective at eliminating this type of fault, which is often intermittent, develops randomly and may occur over a long period.
Several studies have concluded that there is no effective, economical, readily applicable method of verifying the condition of the insulation once the cables have been installed in ducts or directly buried. Dielectric tests are not effective for detecting broken insulation in a dry environment for example in cable ducts. Therefore to eliminate this problem a method is clearly needed to verify the mechanical integrity of the electrical insulation of cables particularly when installed in ducts before the circuit is commissioned. Hydro-Québec decided to develop a new test technique in collaboration with partners Dow Chemical and ndb Technologies.
The Pneumatic Test
The principle of the new insulation test technique for low-voltage cables consists of injecting compressed air into the cable core and then monitoring the pressure changes over a definite time period. A break in the insulation creates a leak, which is then detected by the drop in air pressure inside the cable. Based on this principle, an original test procedure was developed that optimizes the test duration and reliability of the diagnosis. The test is a two-step process: air injection and diagnosis. Compressed air p1is first injected at one end of the cable until the pressure p2at the other end starts rising. The time required for pressure p2starting to rise is specific to each cable. It increases with cable length and strand compaction, and decreases with conductor size. Then compressed air is injected simultaneously at both ends of the cable. The diagnosis “leak/no leak” is based on the pressure drop over time, which is monitored from the moment air injection ends. The test duration is a function of the specific time of each cable that is detected at the first step. This technique is applicable to bundled cables with several phase conductors that can be connected in a loop. However, a procedure for single-phase cables also is available.
The advantage of connecting several cables in a loop is that the ends of the loop are in the same location, making it possible to inject air and measure the pressure at both ends. In addition, all phases can be checked simultaneously, which considerably reduces the time required to perform the test.
Cable Insulation Test Equipment
The cable insulation test equipment, named INSULEAK, is based on a time-optimized method, which was developed as a fully automatic, mobile, stand-alone unit. There are three key components of the test equipment:
- An electropneumatic system for injecting compressed air into the cable core
- An electronic controller with software that supports intelligent test functions
- Pneumatic tubing with cable connectors to connect the tester to cables.
These test components fit into a case, making the unit compact, portable and easy to use. Electric power is supplied by a rechargeable battery housed in the case. However, compressed air at ≈500 kPa (≈70 psi) must be supplied by an external source like a portable or truck-mounted compressor. This pressure level does not affect the cable insulation.
The cable insulation test equipment conducts the test automatically, which only displays the leak/no leak status and does not require any input of the cable parameters. A manual mode allows the user to control the valves for special tests.
Particular care was exercised in developing the universal cable connectors. They were designed in reinforced rubber, with the inside conical and the outside cylindrical. This shape enables the cable connector to mate with cables from 8 mm to 34 mm (0.31 inch to 1.34 inches or 2 AWG to 1,000 kcmil) in diameter. A collar clamp with a smooth inner strip provides a well-sealed fitting-cable connection.
Numerous laboratory and in situ tests have demonstrated that the INSULEAK cable insulation tester achieves the following performance:
- Detection of cable sheath damage of holes less than 1 mm (0.04 inches) in diameter on cables up to 200 m (656 ft) in length.
- Insulation testing takes from 1 minute to a few minutes, primarily depending on the cable cross-sectional area and length that is adjusted automatically according to the pneumatic impedance of the cable being tested (for example, 1 minute for 750 kcmil cable that is 30 m (98 ft) long and 10 minutes for 3/0 AWG cable that is 200 m long).
- Results are unaffected by ambient temperature and air humidity.
- Dielectric properties of cable insulation remain unchanged with air injection pressure up to 500 kPa (≈70 psi).
- Complete tests last from 15 to 20 minutes on average, including the time to taken to install the cable connectors.
The pneumatic tester is simple to use, requires no data entry and displays a clear result: insulation broken or good. A field crew operative can perform the test after a few hours of training.
Field tests on approximately 70 low-voltage cables installed by pulling through ducts on the Hydro-Québec underground cable network showed the relevance of installation quality control with INSULEAK. Even with well-trained crews, insulation damage may occur and go undetected. For example, on one of the five cables on which the INSULEAK tester was used, a perforation was detected, even though every precaution was taken during installation. This 750 kcmil, 40-m (131-ft)-long cable was probably damaged during installation. Once this cable had been removed, a perforation was found in the cable sheath approximately 10 m (33 ft) from one end. The leak though the cable core was quite major. Dielectric tests on the cable before it was removed failed to detect this defect because the environment at that time was not sufficiently conductive.
The in situ cable testing proved the INSULEAK tester is an effective tool for verifying the mechanical integrity of cable insulation, being the only technique for detecting defects in a dry environment where dielectric tests are ineffective. With automatic test and readout functions, it is able to verify and ensure quality control of low-voltage cable installation procedure, thereby preventing arcing faults, improving field crew and public safety, and increasing network reliability.
Even though this technology was developed for cables installed in ducts, it also can be applied to direct-buried cables. It even can be adapted for cables differing in construction from those installed on Hydro-Québec’s underground cable distribution network (for example, to check the mechanical integrity of the sheath of a multicore low-voltage cable used in several European countries or to conduct a test with connectors installed on cable ends).
Use of the pneumatic test technique is a cost-effective alternative to installing low-voltage cables with sophisticated design characteristics such as self-repairing, self-healing or armored cables. By employing the practice of pneumatic testing of cables following installation, the cost may be less than buying cables with more robust insulation (for example, two insulation layers or thicker insulation). Ultimately, the pneumatic test guarantees insulation integrity; there is no such assurance even when cables with ruggedized designs are used.
Based on a comparison of the alternative technologies available and supported by the conclusive in situ test results, Hydro-Québec decided to implement the INSULEAK tester, manufactured by ndb Technologies, for use on the utility’s low-voltage underground cable distribution network. This equipment will be used as a quality-assurance tool for newly installed low-voltage cables.
The authors wish to acknowledge the support and technical assistance of David Larochelle, director of engineering at ndb Technologies Inc., in the preparation of this article.
Janislaw Tarnowski([email protected]) graduated from the faculty of mechanical engineering at the Technical University of Gdansk, Poland, where he received his Ph.D. in 1977. Until 1990, he held full-time academic positions in Poland and Algeria. In 1991, Tarnowski joined Hydro-Québec as a senior scientist at its research institute. His areas of expertise are mechanical engineering aspects related to overhead lines and underground cables. He is member of CIGRÉ Working Group B1-34 on mechanical forces in large-section cable systems.
Jacques Côté([email protected]) received a BSEEdegree in 1988 at Université Laval and a MSEE in 1992 at École Polytechnique de Montreal. He is responsible for construction and maintenance standards at Hydro-Québec Distribution and specializes in underground cables. Côté is involved in the IEEE Insulated Conductors Committee; is a member of the Cable Engineering Committee of AEIC and a voting member for the Canadian Electrical Code, Part 1, representing the Canadian Electricity Association; and chairs the Technical Committee on Underground Systems, CSA C22.3 No 7.
André Gaudreau([email protected]) studied electrical engineering at Université de Sherbrooke, Canada, and joined Hydro-Québec’s research institute in 1993. He has been involved in the study of arcing faults, transformers, lightning and grounding on distribution systems. Gaudreau, who has also worked on modeling various distribution circuit phenomena, is a member of the Ordre des Ingénieurs du Québec.
Dow Chemical| www.dow.com
ndb Technologies| www.ndbtech.com